Generally, chemical mechanical polishing (CMP) is a high precision/mirrored surface polishing method used to obtain global planarization in a semiconductor device manufacturing process. In accordance with such CMP, a slurry is supplied between a polishing pad and a wafer to be polished, so as to chemically etch the surface of the wafer. Using the polishing pad, the etched surface of the wafer is mechanically polished.
Referring to FIG. 1, a typical CMP machine, which is denoted by the reference numeral 1, is schematically illustrated. Also, a CMP method using the CMP machine 1 is schematically illustrated in FIG. 2. The CMP method includes a chemical etching reaction process and a mechanical polishing process, which are conducted using a polishing pad 10 included in the CMP machine 1. The chemical etching reaction is carried out by a slurry 42. That is, the slurry 42 serves to chemically react with the surface of a wafer 30 to be polished, thereby making it possible for the mechanical polishing process, following the chemical etching reaction, to be easily carried out. In the mechanical polishing process, the polishing pad 10, which is fixedly mounted on a platen 20, rotates. The wafer 30, which is firmly held by a retainer ring 32, rotates while oscillating. A slurry containing abrasive particles is supplied to the polishing pad 10 by a slurry supply means 40. The supplied slurry is introduced between the polishing pad 10 and the wafer 30. The introduced abrasive particles come into frictional contact with the wafer 30 by virtue of a relative rotating speed difference between the polishing pad 10 and the wafer 30, so that they conduct mechanical polishing. The slurry 42 is a colloidal liquid containing abrasive particles having a grain size of nanometers. This slurry 42 is spread on the polishing pad 10 during the polishing process. As the polishing pad 10 rotates during the polishing process, the slurry 42 supplied to the polishing pad 10 is outwardly discharged from the periphery of the polishing pad 10 due to a centrifugal force caused by the rotation of the polishing pad 10. In order to achieve an enhanced polishing efficiency, many abrasive particles should remain for a desirable lengthy period of time on the upper surface of the polishing pad 10 so that they. participate in the polishing of the wafer. That is, the polishing pad 10 should make the slurry 42 be held on the surface thereof for as long a period of time as possible.
Centrifugal force generated during the rotation of the CMP pad is higher at a position nearer to the periphery of the polishing pad. Due to such a centrifugal force difference between different radial positions on the polishing pad, the slurry on the polishing pad exhibits an increased flow rate as it approaches the periphery of the polishing pad. Thus, the slurry is non-uniformly distributed in the radial direction of the polishing pad. Due to such a non-uniform distribution of the slurry, the wafer is non-uniformly polished because its polishing rate is varied depending on a radial position of the polishing pad in contact with the wafer's surface. Such a variation in polishing rate affects the planarization of the wafer. As a result, the polishing pad exhibits a considerable difference in polishing rate between its central portion and its peripheral portion. For this reason, it is necessary to uniformly distribute the slurry over the polishing pad by controlling the flow of slurry on the polishing pad.
During the polishing process, the wafer is pressed against the polishing pad so that it comes into frictional contact with abrasive particles. Due to this pressure, however, it may be difficult for the slurry to reach the central portion of the wafer. For this reason, the slurry may be distributed at the central portion of the wafer in a relatively reduced amount, as compared to the amount at the peripheral portion of the wafer. As a result, the wafer is non-uniformly polished.
In order to solve such a problem, a method has been proposed, in which holes or grooves having a desired width, depth and shape are formed on a CMP pad. Such holes or grooves act to control the flow and distribution of the slurry continuously supplied during the polishing process.
Now, holes or grooves conventionally formed on a polishing pad will be described in conjunction with the annexed drawings.
FIG. 3a is a schematic view illustrating a polishing pad formed with grooves respectively having the form of concentric circles. FIG. 3b is a cross-sectional view taken along the line A—A of FIG. 3a. As shown in FIGS. 3a and 3b, the grooves formed on the polishing pad have the form of concentric circles uniformly spaced apart from one another in a radial direction while having different diameters, respectively. The slurry, which is continuously supplied onto the polishing pad, is forced to move outwardly by a centrifugal force generated as the polishing pad rotates. As a result, during the polishing process, the slurry is temporarily collected in the concentric circular grooves, and then outwardly discharged from those grooves. An example of such concentric circular grooves is disclosed in U.S. Pat. No. 5,984,769. This U.S. Pat. No. 5,984,769 discloses a polishing pad formed with a plurality of concentric circular grooves or spiral grooves. The polishing pad is divided into a plurality of groove regions where grooves having different widths and different lengths are formed, respectively. However, such a polishing pad having circular or spiral grooves cannot meet various conditions required in a CMP process because it has a fixed pattern formed only of grooves.
FIG. 4a is a schematic view illustrating a polishing pad formed with holes having a conventional structure. FIG. 4b is a cross-sectional view illustrating holes shown in FIG. 4a. In the polishing pad of FIG. 4a, a plurality of holes are regularly arranged. The holes arranged on the polishing pad store a slurry supplied thereto, thereby retarding a discharge of the stored slurry caused by centrifugal force. U.S. Pat. No. 5,853,317 discloses a polishing pad formed with grooves at a first surface thereof serving as a polishing surface to polish an object and grooves at a second surface fixedly mounted to a platen in such a fashion that the grooves formed at the second surface have a larger size than the grooves formed at the first surface. Also, U.S. Pat. No. 5,329,734 discloses a polishing pad having a first region formed with a plurality of pores and a second region formed with a plurality of openings.
In the case of the conventional polishing pad having grooves or holes uniformly spaced apart from one another, the slurry supplied onto the polishing pad is hindered from flowing toward the central portion of a wafer being polished at regions where the polishing pad is in contact with the wafer. As a result, a degradation in polishing rate occurs at the central portion of the wafer.
On the other hand, in the case of the conventional polishing pad having concentric circular grooves, a superior slurry storage capacity is obtained because each groove has a partially closed structure having vertical groove walls capable of retaining the slurry in the groove against centrifugal force, as compared to other conventional polishing pads. However, this polishing pad has a drawback in that each groove has an insufficient depth corresponding to ¼ of the thickness of the polishing pad.
Since the grooves or holes formed on the above mentioned conventional polishing pads have a fixed pattern such as concentric circles or a lattice, it is difficult to form a groove pattern capable of effectively controlling the flow of a slurry. In accordance with the method for forming perforations using perforating pins, the perforations have a fixed shape. Since the holes have a simple and fixed pattern, it is difficult to arrange them to have diverse hole patterns desired in a CMP process.
In order to solve such problems, accordingly, it is necessary to design the shape, density and distribution of grooves or holes, taking into consideration the given polishing process conditions such as centrifugal force and wafer position.